Recurrent UTI

I have had about 8 uti's in the past year and I am seeing a urogynecologist for them.  He has me on methenamine, 2x daily, which has seemed to work..been on it about 3 months.  Over the weekend I was fine, but woke Monday morning to all the symptoms. I think I used the bathroom about 7 times in one hour.  Pain upon stopping urinating, cloudy urine, and kind of like bladder/urethra spasms upon emptying  my bladder. I hadn't had a lot of water over the weekend so I started drinking water as soon as I got up...and took my methenamine and the little pills that are supposed to help with the pain.  By the time I got to labcorp I probably had had about 3 pints of water.  I called my doctor and based on my history, he called in a script which I got about 4 hours later.  I have been taking them since Monday night.  The doctor's office called yesterday and said the urine culture came back negative, and to stop taking the antibiotic (macrobid).  I KNOW what a uti feels like...is it possible that I diluted down my urine culture so much that an infection wouldn't show on a culture?  I decided to finish the antibiotic as I really feel it was a uti and I want to make sure I get rid of it.  Thanks for any insight!